docs: def label(self) -> str:
          """A latex formatted label representing axis expression."""
          label = self.expression.replace("_", "\\;")
          if self.units_kind:
              symbol = wt_units.get_symbol(self.units)
              for v in self.variables:
                  vl = "%s_{%s}" % (symbol, v.label)
                  vl = vl.replace("_{}", "")  # label can be empty, no empty subscripts
                  label = label.replace(v.natural_name, vl)
              units_dictionary = getattr(wt_units, self.units_kind)
              label += r"\,"
              label += r"\left("
              label += units_dictionary[self.units][2]
              label += r"\right)"
          label = r"$\mathsf{%s}$" % label
          return label